This invention relates to the combustion of coal and in particular to the reduction of mercury (Hg) in flue gases generated during coal combustion.
Mercury is a constituent part of coal mineral matter. Mercury volatizes during coal combustion as elemental mercury (Hg0). Mercury that remains as elemental mercury through the furnace tends to remain in the flue gas. It is desirable to lower the amount of mercury released in flue gases during coal combustion.
Oxidized mercury is more easily collected by emission control devices than is elemental mercury. Oxidization of mercury in flue gases is a known technique to capture mercury and remove it from flue gases. As flue gases cool, mercury is partially oxidized by chlorine which is present in coal and is released during combustion. It is believed that most oxidized mercury (Hg+2) in flue gas is present as mercury chloride (HgCl2). Oxidation of mercury occurs in combustion gas-phase reactions and on the surface of fly ash. It is believed that mercury oxidation on the surface of fly ash is a predominant channel of mercury oxidation.
Oxidized mercury (HgCl2 or Hg+2) is water soluble and is easily adsorbed on high carbon fly ash or activated carbon. The mercury captured by fly ash may be collected with the ash and removed via a particulate collection system. Oxidized mercury is also easily removed by wet scrubbers that are used to control sulfur dioxide (SO2) emissions. Mercury control is generally most effective when the mercury in flue gas is mostly oxidized.
Bituminous coals typically have high chlorine content which improves mercury oxidation. In addition, fly ash of bituminous has a relatively high carbon content which promotes good mercury oxidation. In contrast, low rank coals have a relatively low chlorine content and have a high reactivity that results in low carbon content in fly ash. Accordingly, elemental mercury from the combustion of low rank coals oxidizes to a lesser extent than does mercury from the combustion of bituminous or other high chlorine coals.